1. Field of the Invention
This invention relates to a copolyester film and a hot melt adhesive of said film.
2. Description of the Prior Art
In recent years, hot melt adhesives have been used in increasing quantities in view of their non-polluting, energy-saving and resource-conservation characteristics. Hot melt adhesives of the film type, in particular, are known to be advantageous in that they make for uniformity of thickness of the adhesive layer and a stable and uniform bond strength, as well as in that as the desired adhesion can be established by mere melting of the superficial layer of the film, they lend themselves well to a short-time bonding process and require a minimum of bonding heat. Moreover, the thermal degradation of the adhesive is negligible.
Hot melt adhesives of the polyester type are well known. These adhesives are excellent in heat- and cold-resistance, thermal stability, electrical properties, weather resistance, moisture resistance, resistance to chemicals and safety in food use, and have therefore been used widely in such applications as electrical parts assembly, automotive parts assembly, textiles, can manufacture, etc.
In regard to polyester hot melt adhesives, many proposals have heretofore been made. For example, a hot melt adhesive of polyesters derived from a selected dicarboxylic acid component and a selected diol component having a special structural formula as the polyester materials is well known. Also known is a hot melt adhesive of polymers having a carboxylic acid amide, carboxylic acid anhydride, carboxylic acid ester, urethane or urea bond either in the backbone chain or in the side chain (Japanese Patent Publication No. 22750/82). It is, thus, possible to improve a given property by using selected components or a selected combination of components, but such an attempt tends to result in changes in other physical properties.
U.S. Pat. No. 4,143,790 discloses a polyester hot melt adhesive wherein at least 45 mole % of the dicarboxylic acid component is accounted for by terephthalic acid and at least 55 mole % of the diol component is accounted for by 1,4-butanediol and its relative viscosity and degree of crystallinity are in the range of 1.2 to 1.8 and in the range of 0 to 30 percent, respectively. U.S. Pat. No. 3,515,628 teaches a polyester copolymer-based hot melt adhesive such that its dibasic acid component is a mixture of terephthalic acid and an aliphatic dicarboxylic acid and its glycol component is either 1,4-butanediol or a mixture of 1,4-butanediol and ethylene glycol, neopentyl glycol or the like, with a glass transition point not higher than 30.degree. C. and a degree of crystallinity not exceeding 25%. U.S. Pat. No. 4,335,188 describes a copolyester hot melt adhesive in which at least 85 mole % of its dicarboxylic acid component is terephthalic acid and its diol component is a mixture of 97 to 65 mole % of 1,4-butanediol and 3 to 35 mole % of polyethylene glycol having a molecular weight in the range of 150 to 550. Further, Japanese Patent Application Laid-Open No. 78234/84 discloses a hot melt adhesive comprising a polyester copolymer consisting of a dibasic acid component made from 70 to 90 mole % of terephthalic acid and the balance of other dibasic acids, 70 to 97 mole % of 1,4-butanediol and 3 to 30 mole % of diethylene glycol, with a reduced viscosity of at least 0.6 dl/g, a glass transition point not less than 31.degree. C. and a degree of crystallinity in the range of 15 to 40%.
However, as the scope of their application was expanded, the performance characteristics required of hot melt adhesives became more diversified or critical. The characteristics required include the following.
(a) High bonding strength with respect to substrates PA0 (b) Good balance between tensile shear bonding strength and peel strength PA0 (c) Adaptability to high-speed bonding procedures PA0 (d) Good bonding workability PA0 (e) Durability under varied conditions PA0 (f) Stable quality PA0 (g) Adaptability to food packaging